This invention relates to apparatus for anaerobic digestion of biodegradable waste material.
In the design and operation of anaerobic digestion apparatus, mixing of the contents of the tanks, which are often very large, is necessary to ensure good contact between the bacteria and their food supply which is the effluent, i.e. biodegradable waste material, to be treated.
The concentration of the solids material which should be maintained in suspension in the tank during anaerobic digestion is often such, especially during start-up of the process, that the solids material tends to settle fast to the floor of the tank where it can compact and require a large force to disturb it and move it back into suspension in the tank. Because of this much more energy is required to maintain in suspension a low concentration of solids than is required with a high concentration of solids which behaves more like a homogeneous liquid and can be kept mixed by using relatively inexpensive gas lift pumps which use relatively little power.
In order for gas lift pumps to produce uniform liquid velocitites of the order of 0.5 to 1.5 feet per second over the whole floor of the large tank, very large energy inputs are required to overcome the friction and hydraulic shock losses of moving these very large volumes.
An alternative method of preventing settling and compaction of the solids material as a sludge on the bottom of the tank is to use a rotating scraper arm. Such scraper arms are usually driven by a central torque tube from outside the tank roof by a reduction gear drive system. For large tanks the central drive systems for the scraper arm involve very large torques and are consequently very expensive.
The difficulty of mixing the anaerobic bacteria in the waste material in the digestion tank is one of the important factors currently limiting the diameter of the digestion tanks which, for this reason, are built higher and frequently involve very costly foundations.